1. Field of the Invention
The present invention relates generally to audio/video (AV) systems, and more specifically, to a method and system for maintaining an ideal frequency ratio between numerically-controlled frequency sources.
2. Background of the Invention
Numerically controlled oscillators and other digital phase/frequency timebases are increasingly used in digital circuits that receive, generate and transform data to and from analog devices. In particular, the above-incorporated patent applications describe video and audio systems and circuits that sample input signals and transform them to the digital domain.
When two or more information streams are to be synchronized to numerically-generated timebases, the finite expression of the number controlling each timebase produces an error between an ideal relationship of the frequencies of the timebases and the actual frequencies of the timebases as controlled by the input numbers. In the above-incorporated patent applications, techniques are described for generating timebase information and/or synchronization signals from numbers representing ratios between a local precision reference oscillator frequency and a known or detected input information reference frequency and also from ratios between various desired output frequencies and the input information frequencies.
When two or more timebase or clock signal outputs are generated in such a scheme, for example in the audio/video synchronizing embodiment described in the above incorporated application entitled “METHOD AND SYSTEM FOR VIDEO-SYNCHRONOUS AUDIO CLOCK GENERATION FROM AN ASYNCHRONOUSLY SAMPLED VIDEO SIGNAL”, long-term error between the generated audio clock or timebase and the generated video clock or timebase occurs due to the finite expression of the audio ratio and the video ratio numbers. There are two sources of finite expression error present in the described circuits: 1) The generated video synchronization reference is only as accurate as the video expression number will permit; and 2) The audio synchronization reference is not generated from the actual generated video synchronization reference and further is only as accurate as the audio ratio precision permits. Although these errors are small if the number of bits used to represent the ratios is sufficiently high, over long periods of time, synchronization drift will still occur between the sampled video and the sampled audio.
Therefore, it would be desirable to provide a method and system for maintaining an ideal ratio between numerically-controlled frequency sources so that long-term drift between the frequency sources can be eliminated.